1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system.
2. Related Background Art
Conventionally, in an image forming apparatus adopting an electrophotographic system, a laser printer and the like have been known. Description will be given as to a fixing apparatus provided to a prior art image forming apparatus.
In the fixing apparatus provided to the prior art image forming apparatus, a heat fixing apparatus is often used in order perform fixing processing on transferring paper which is a recording medium for bearing a toner image as an unfixed image. That is because the heat fixing apparatus has many merits. For example, the heat fixing apparatus is superior in the storage stability, capable of supporting various devices from a low-speed device to a high-speed device, and has many options of toner. Further, there has been conventionally known a fixing apparatus configured to heat a pressure roller from the inside in order to prevent the pressure roller as a pressure body from being fouled.
Here, a heat roller fixing apparatus as an example of such a fixing apparatus will be described in detail based on FIG. 7.
FIG. 7 is a cross-sectional view showing a schematic structure of a heat roller fixing apparatus.
The heat roller fixing apparatus includes: a fixing roller 121 as a fixing body; a pressure roller 122 as a pressure body; a heater 135(a) as fixing body heating means which is a halogen lamp as a heating source for the fixing roller 121; a heater 135(c) as pressure body heating means which is a halogen lamp as a heating source for the pressure roller 122; temperature adjusting thermistors 129 and 130 as temperature detecting means for the fixing roller 121 and the pressure roller 122; a release claw 125; and a fixing inlet guide 134. In this example, the both temperature adjusting thermistors 129 and 130 are provided in non-image areas. It is to be noted that the temperature adjusting thermistor 130 is used for a non-printing surface side and it may be hence provided at the center.
The fixing roller 121 is configured to have a primer layer (not shown) as an adhesive layer on a hollow core metal 123 made of aluminium, iron or stainless steel and further a fluorocarbon resin layer 124 on the primer layer. As to a thickness of the primer layer (not shown), 5 to 20 xcexcm is appropriate, and 30 to 70 xcexcm is appropriate for a thickness of the fluorocarbon resin layer 124. This fluorocarbon resin layer 124 may be formed by using a tube consisting of PFA resin or by baking the PFA resin.
Since the fixing roller 121 is constituted attaching a high value on the mold releasing property of the surface, the fluorocarbon resin is constituted by pure PFA resin in which a filler is not mixed. A silicon rubber layer may be provided between the fluorocarbon resin layer 124 and the core metal 123 according to needs.
The pressure roller 122 has a structure that an elastic body layer 127 consisting of silicon rubber having the heat resistance is bonded on a hollow core metal 126 consisting of aluminium, iron or stainless steel through a primer layer (not shown) and a PFA resin tube 128 is applied on the uppermost layer portion. As this PFA resin tube 128, there is used a material in which carbon is mixed so that the resistance value becomes 104 to 1012 xcexa9xc2x7cm in order to increase the bias effect from the pressure roller.
The heaters 135(a) and 135(c) as heating sources for the fixing roller 121 and the pressure roller 122 are provided inside the fixing roller 121 and the pressure roller 122, respectively, and the heater 135(a) and the heater 135(c) are configured to be driven by the same power supply circuit as controlling means. By using one drive circuit for the heater 135(a) and the heater 135(c), the heater 135(a) in the fixing roller 121 and the heater 135(c) in the pressure roller 122 can be alternatively energized in accordance with each control unit (in this example 1000 ms) which is a predetermined period in time with a predetermined time length ratio (which will be referred to as a lighting ratio hereinafter), and more power can be supplied to the fixing roller 121 provided on a printing surface side (side carrying the toner 132) of the transferring material to be subjected to fixation, thereby avoiding the power loss caused due to a switching time of a switching device.
Here, FIG. 8 shows a heater lighting result when an A4-size landscape sheet and an LGL-size sheet are supplied.
As shown in FIG. 8, in case of feeding the A4-size landscape sheet, the heater 135(a) is in the on state with the time length of 80% in the control unit in order to maintain the fixing roller 121 at a fixed temperature. Since the heater 135(a) and the heater 135(c) are alternatively turned on, the heater 135(c) is in the on state only 20%. On the contrary, in case of feeding the LGL-size sheet, the heater 135(a) is in the on state only 40% in order to maintain the fixing roller 121 constant, and the heater 135(c) is in the on state 60%. The paper size and the heater lighting ratio of each roller are as shown in Table 1.
When the heater 135(a) is not turned on, the high fixing property can be satisfied by heating the pressure roller 122 from the inside. In the above-described structure, both the hardness and the wall thickness of the pressure roller 122 are restricted in order to maintain a nip width to a fixed value or a higher value.
However, the above-described conventional structure/controller have the following drawbacks.
In the structure mentioned above, since the wall thickness of the pressure roller is large, a considerable difference in temperature is generated between the core metal temperature and the surface temperature of the pressure roller depending on the lighting ratio of the heater for the pressure roller. The heater control is determined based on a temperature of the fixing roller on the printing surface side on which the fixing property greatly depends. Therefore, when the power size is small, excessive power is supplied to the heater for the pressure roller, thereby increasing the temperature of the core metal of the pressure roller. As a result, the primer between the core metal and the rubber is deteriorated, and the pressure roller is damaged.
Therefore, it is an object of the present invention to provide a fixing apparatus and an image forming apparatus having this fixing apparatus capable of avoiding thermal degradation of a pressure body due to excessive increase in temperature of a core metal of the pressure body.
To achieve this aim, according to the present invention, there is provided a fixing apparatus or an image forming apparatus, comprising: a fixing body and a pressure body which are pressed against each other and rotate; fixing body heating means for heating the fixing body upon receiving supply of power from a power supply; pressure body heating means for heating the pressure body upon receiving supply of power from the power supply; temperature detecting means for detecting a temperature of at least one of the fixing body and the pressure body; and controlling means for controlling power supplied to the fixing body heating means and the pressure body heating means from the power supply in such a manner that a temperature detected by the temperature detecting means becomes a predetermined set temperature, an unfixed image being fixed on a recording medium by heating and pressing the recording medium bearing the unfixed image while pinching and conveying the recording medium, the pressure body having a core metal, an elastic body layer covering an outer periphery of the core metal, and a mold releasing layer covering an outer periphery of the elastic body layer, wherein the controlling means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to be equal to or lower than a predetermined quantity in such a manner that a temperature of the core metal of the pressure body is maintained at a predetermined temperature or a lower temperature.
According to the present invention, since the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means in such a manner that a temperature of the core metal of the pressure body is maintained at a predetermined temperature or a lower temperature, it is possible to avoid thermal degradation of the pressure body due to excessive increase in temperature of the core metal of the pressure body.
Preferably, a power supply circuit for the fixing body heating means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to a predetermined quantity or a lower quantity in such a manner that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature. Since the power supply circuit for the fixing body heating means sets the electric energy per predetermined time supplied from the power supply to the pressure body heating means to a predetermined quantity or a lower quantity in such a manner that a temperature of the core metal of the pressure body is set at a predetermined temperature or a lower temperature, it is possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.
Preferably, by the controlling means, the fixing body heating means and the pressure body heating means are driven by the same power supply (source) circuit.
Preferably, with the controlling means, by setting a value of a voltage supplied form the power supply to the pressure body heating means so as to be small as a width of the recording medium to be subjected to fixation in a direction orthogonal to a conveying direction becomes small, the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity. Since a value of the voltage from the power supply to the pressure body heating means can be set smaller by the controlling means as the width of the recording medium to be subjected to fixation becomes small, it is possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.
Preferably, the temperature detecting means detects a temperature of the pressure body. The temperature detecting means detects a temperature of the pressure body, and the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means so that a temperature of the core metal of the pressure body can be maintained at a value not more than a predetermine value based on the above-mentioned temperature, thereby avoiding thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.
Preferably, the temperature detecting means detects a temperature of the core metal of the pressure body. The temperature detecting means detects a temperature of the core metal of the pressure body, and the electric energy per predetermined time supplied from the power supply to the pressure body heating means is set to a predetermined quantity or a lower quantity by the controlling means so that the above-described temperature can be maintained at a predetermined temperature or a lower temperature, thereby preventing thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.
Preferably, the fixing body heating means is constituted by two fixing body heating bodies having luminous intensity distributions different from each other, which are provided inside the fixing body and generate heat by power from the power supply. The pressure body heating means is constituted by one pressure body heating body which is provided inside the pressure body and generates heat by power from the power supply. Any one of the two fixing body heating bodies and the pressure body heating body are driven by the same power supply circuit. By using the power supply circuit for one fixing body heating body, the electric energy per predetermined time supplied from the power supply to the pressure body heating body is set to a predetermined quantity or a lower quantity so that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature, thereby preventing thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.
Preferably, with the controlling means, a set temperature for the pressure body heating means is determined so as to be small as the width of the recording medium to be subjected to fixation in a direction orthogonal to a conveying direction becomes smaller, thus setting the electric energy per predetermined time supplied from the power supply to the heating body heating means to a predetermined quantity or a lower quantity. By the controlling means, a value of the voltage from the power supply to the pressure body heating means is set so as to be small as the width of the recording medium to be subjected to fixation is decreased so that a temperature of the core metal of the pressure body can be maintained at a predetermined temperature or a lower temperature, and it is hence possible to prevent thermal degradation of the pressure body caused due to excessive increase in temperature of the core metal of the pressure body.